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GENEVA — The United Nation’s weather organization says greenhouse gas levels in the atmosphere climbed to a new record level last year, warning that ‘‘the window of opportunity’’ to act against climate change is nearly closed.

The World Meteorological Organization says globally-averaged concentrations of carbon-dioxide reached 405.5 parts per million in 2017, up slightly from the previous year and from 400.1 in 2015. Concentrations of other heat-trapping gases, like methane and nitrous oxide, also rose.

WMO says it’s the first time that consolidated figures for 2017 have been available.

WMO Secretary-General Petteri Taalas said the last time Earth had such high levels of CO2 concentration was at least 3 million years ago.

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Without cuts in greenhouse gases, he said, ‘‘climate change will have increasingly destructive and irreversible impacts on life on Earth.’’

The U.N.’s Intergovernmental Panel on Climate Change issued a report in Oct. 2018 that detailed how Earth’s weather, health, and ecosystems would be in better shape if the world’s leaders could somehow limit future human-caused warming to just 0.9 degrees Fahrenheit from now, instead of the globally agreed-upon goal of 1.8 degrees.

‘‘For some people this is a life-or-death situation without a doubt,’’ Cornell University climate scientist Natalie Mahowald, a lead author on the report, said at the time.

In October, the U.N. panel said meeting the more ambitious goal of slightly less warming would require immediate, draconian cuts in emissions of heat-trapping gases and dramatic changes in the energy field. While that’s technically possible, at the time it saw little chance of the needed adjustments happening.